Chapter 10 - Exponents and Radicals - Study Summary - Practice Exercises - Page 692: 20

$-3-12i$

Adding the real parts and the imaginary parts, the given expression, $ (5-3i)+(-8-9i) ,$ simplifies to \begin{array}{l}\require{cancel} (5-3i)+(-8-9i) \\\\= 5-3i-8-9i \\\\= (5-8)+(-3i-9i) \\\\= -3-12i .\end{array}